This invention relates to new and useful reinforced composite membranes for use in conveying, material handling, surface modifying, surface protection, and barrier applications, in which the two opposing faces of the membrane differ significantly in composition and physical characteristics, each face being constructed to perform independent functions in a given single application, thereby optimizing overall performance.
More specifically, this invention relates to reinforced composite membranes in which one face is a perfluoropolymer, such as polytetrafluoroethylene (PTFE), and the other face is an elastomer. Although the use of elastomeric materials of varied compositions is contemplated, silicone rubber is the preferred elastomeric component.
Polytetrafluoroethylene (PTFE) coated fiberglass fabrics and silicone rubber coated fiberglass fabrics are examples of reinforced composite membranes commonly used in many of the applications mentioned above. The two materials share several unique and valuable physical properties: flexibility, thermal stability in operating environments exceeding 350° F., and low-energy surfaces providing easy release to sticky, viscous, or adhesive materials. On the other hand, they may differ markedly in surface hardness, finish, frictional characteristics, and surface qualities difficult to specify but related to the way the surface adheres to other surfaces. PTFE has one of the lowest coefficients of friction possessed by any common material and exhibits minimal “stick-slip” behavior. On the other hand, silicone rubber, depending on its composition, finish, and hardness (durometer), often has the high coefficient of friction and pronounced stick-slip behavior or “grabby” quality typically associated with elastomeric materials.
The choice of whether to use a PTFE or a silicone rubber composite in a given application sometimes involves consideration of the materials' frictional and related surface characteristics. Certain applications may require a material with a low coefficient of friction, in which case PTFE composites would be expected to perform very well, while silicone rubber constructions would not. In other applications, a material with a high coefficient of friction or stick-slip characteristics may be required, in which cases a silicone rubber material would answer readily, while a PTFE construction would not.
However, in some applications, a membrane with the frictional and related surface characteristics of PTFE on one face and those of an elastomer on the other face may be needed. To address this need, efforts have been made to combine the two materials in a double-faced membrane, with PTFE on one face and silicone rubber on the opposite face. In the past, these attempts have yielded materials with a strong tendency to curl, making their handling extremely difficult and limiting their usefulness. The curling tendency is due to imbalanced stresses generated in manufacturing these composites, the result of differences in the curing characteristics, thermal coefficients of expansion, and modulae of the two components. It is the object of this invention to produce double-faced PTFE-elastomer reinforced composite membranes with curling tendency controlled to the extent that their handling characteristics and usefulness remain uncompromised.